MHB Supeerstar's questions at Yahoo Answers regarding optimizing quadratic functions

AI Thread Summary
The discussion revolves around solving quadratic problems related to maximizing the area of a rectangle and finding minimum and maximum values of a quadratic function. For the rectangle problem, the maximum area is determined to be 3 square units, achieved when x equals 3/2. The minimum value of the quadratic function 3x^2 + 7x - 2, within the interval -3 ≤ x ≤ 0, is calculated to be -73/12. Conversely, the maximum value of the same function within the interval 0 ≤ x ≤ 3 is found to be 46. The solutions highlight the use of the axis of symmetry in determining these values.
MarkFL
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MHB
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Here are the questions:

Help with solving these quadratic worded problems?


1) A rectangle is constructed so that one vertex is at the origin, and another vertex is on the graph of y=3 - 2x/3 where x >0 and y>0 and adjacent sides are on the axes. what is the maximum possible area of the rectangle?

2) What is the minimum value of 3x^2 +7x -2 if -3 ≤ x ≤ 0 ?

3) What is the maximum value of 3x^2 + 7x - 2 if 0 ≤ x ≤ 3?

I have posted a link there to this topic so the OP can see my work.
 
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Hello supeerstar,

1.) Let's look at a plot of the given line and a rectangle constructed as instructed:

View attachment 1525

We see the area of the rectangle is:



We also see that we require .

Since we may write:



Observing this is a parabola opening downwards, we know the maximum must occur on the axis of symmetry, given by:



And so we find:



And so the maximum area of the rectangle is 3 square units.

For problems 2.) and 3.), let:



We find the axis of symmetry for the given quadratic is:



Since this is a quadratic opening upwards, we know the global minimum is:



For any interval wholly to the right of the axis of symmetry, we know the maximum value of the function is at the right end-point. Hence:

2.)

3.)
 

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